Light source stabilisation

ABSTRACT

An apparatus for sensing data from a remote optical sensor  16  has its frequency stabilised by balancing the outputs of narrow band filter  28 30,  spaced about a desired frequency  36  positioned at about the 3db down points  40  of a broad band light source  10  using voltage control, current control or temperature control to vary the frequency of the wide band light source  10.  Difference between the outputs through the two narrow band filters  28 30  can be used to drive an amplifier  48  to correct the frequency of the broad band light source. The outputs through the two narrow band filters  28 30  can be converted  52  to binary numbers and fed to a microprocessor  56  which is used, via analog conversion  60,  to drive the amplifier  48.  The broad band light source  10  can be pulse modulated  68  to provide temporally separate light pulses  92 94  through each of the narrow band filters  28 30,  measured at separate times. The corrective output to the amplifier  48  can be governed by a ratio between the outputs through the narrow band filters  28 30  rather than by a difference there between.

The present invention relates to remote monitoring of a parameter whichis encoded by means of the parameter or parameters varying thedimensions of a dual path light cell (interferometer). It particularlyrelates to temperature, flow rate, chemical property, strain or pressuremeasurement using light, sent to and returned from an interferometrictransducer at the end of a fibre optic line.

The present invention most particularly relates to a method andapparatus where the broadband light is sent, via the fibre optic line,to and from the transducer, the return light being subject to a firstintensity measurement in broadband and subjected to a second intensitymeasurement after it has been subjected to narrow bandwidth filtering,the ratio of the two measurements giving a measure of the interferometerpath difference and thus of the measured temperature, flow rate,chemical property, strain or pressure. The invention concernsimprovements to such a measurement system and is applicable to, but notexclusively, for example, pressure, flow rate, chemical property, strainor temperature measurements in a hydrocarbon well or like hostile andinaccessible environment.

The prior art includes a first type of signal processing where the pathimbalance occasioned by the interferometer is determined by submittingthe signal from the sensor to a second (interrogating) interferometer,the path imbalance of which can be adjusted. By sweeping theinterrogating interferometer over the range of path imbalances which canbe exhibited by the sensor, a cross-correlation between the modulationsof the source spectrum applied by the two interferometers is obtainedand, from this and detailed knowledge of the position of theinterrogating interferometer, the position of the sensing interferometercan be deduced, often with a high degree of resolution and absoluteaccuracy. Specific implementations include mechanically scannedinterferometers. Electronically scanned interferometers have beenimplemented by splitting the incoming optical signal from the sensorwith a wedge and applying the resulting Fizeau fringes to a CCD Chargecoupled device array or similar linear image sensor (U.S. Pat. Nos.5,202,939 and 5,392,117). There are many variations on these basicprinciples in the literature.

The benefits of recovering the interferometer position by some form ofmatched interferometer are that a wide dynamic range can be achievedsince in most configurations the transducer can vary the optical pathdifference by more than one fringe. In addition, the information isspread over the entire spectrum of the source and it follows that themethod is robust to variations in the spectral attenuation of the fibreconnecting the sensor to the opto-electronic unit (the down lead).However, the stability of the recovery interferometer is then criticalto the accuracy of the measurement and ensuring adequate stabilityagainst changes in temperature and mechanical drift can result in anexpensive readout system. In the case of electronically scannedinterrogating interferometers, the range of low-cost, line imagingdevices usually restricts operation to wavelengths shorter than 1000 nm.As a result, the fibre losses at the operating wavelength are increased,which precludes operation over distances of many km, a requirement, forexample, in the oil industry.

The exact style of the transducer is not the subject of the presentinvention. The present invention can function with any style of opticaltransducer adapted to provide output indicative of the value of anyparameter.

The present invention seeks to provide ways to improve the stability andaccuracy of measurement using fibre optic transducers, includinginterferometric transducers. For instance in order to ensure theaccuracy of the measurements over time it is important that the centralwavelength of the broad band source is stable relative to the narrowband filter. If it is not stable then any ratio used to calculate themeasurand can drift independently of the actual transducer measurement.

The present invention seeks to provide means for stabilising drift ofthe centre frequency of a broad band light source.

In a first aspect, the present invention consists of an apparatus forprocessing signals from a remote optic sensor, said apparatus includingmeans for supplying a broad band light beam for use in interrogating theoptic sensor, said apparatus comprising: a first narrow band filter anda second narrow band filter, said first and second filters havingrespective centre frequencies spaced about the desired centre frequencyof the broad band light source; coupling means for coupling a firstsample of said broad band light beam through said first narrow bandfilter and a second sample of said broad band light beam through saidsecond narrow band filter; first means to measure the filtered firstsample of said broad band light beam; second means to measure thefiltered second sample of said broad band light beam; comparison meansto compare the outputs of said first means and said second means and toproduce an error indication in response thereto; means to employ saiderror indication to apply a correction to said means to produce saidbroad band light beam to move the central frequency of said broad bandlight beam towards said desired centre frequency.

The invention further provides that the central frequency of the broadband light beam may be moved into co-incidence with said desired centralfrequency.

The invention further provides that the error signal can be a differencesignal.

The invention further provides that the comparison means can beoperative to take the ratio between the output of the first means tomeasure and the output of the second means to measure, and to generate acorrective output error indication if the ratio changes.

The invention further provides that moving the centre frequency of thebroad band beam includes controlling the current through the means forsupplying a broad band light beam.

The invention further provides that moving the centre frequency of thebroad band beam includes controlling voltage applied to the means forsupplying a broad band light beam.

The invention further provides that moving the centre frequency of thebroad band beam includes controlling the temperature of the means forsupplying a broad band light beam.

The invention further provides that the broad band light beam canprovide illumination for an optical sensor on the distal end of a fibreoptic line in a hydrocarbon well.

The invention further provides that the optical sensor can be apressure, flow rate, temperature, chemical property, or strain sensor.

The invention further provides that the means to supply a broad bandlight beam can be substantially temporally continuous and that the firstand second means to measure are also temporally continuous.

The invention further provides that the means to supply a broad bandlight beam can be substantially temporally discontinuous and that thefirst and second means to measure are operative to measure only when thefirst and second samples of the broad band light beam respectively arepresent for measurement.

The invention provides that the first and second means to measurecomprise a single measurement path and means temporally to separatemeasurement activity for said first filtered sample and for said secondfiltered sample.

The invention further provides that the first narrow band filtercomprises a first narrow band reflector, and said second narrow bandfilter comprises a second narrow band reflector, said first and secondnarrow band reflectors being separated by a delay line, for said firstfiltered sample and said second filtered sample temporally to beseparated.

The invention further provides that a means for coupling first andsecond samples of said broad band light beam can be a broadbandreflector, and the measured samples are the pulses being reflected bythe narrowband reflectors from the broadband reflector.

In a second aspect, the present invention consists of a method forstabilizing the broad band light source of system used for processingsignals from a remote optic sensor, including the steps of: supplying abroad band light beam for use in interrogating the optic sensor;providing a first narrow band filter and a second narrow band filter,said first and second filters having respective centre frequenciesspaced about the desired centre frequency of the broad band lightsource; coupling a first sample of said broad band light beam throughsaid first narrow band filter and a second sample of said broad bandlight beam through said second narrow band filter; measuring thefiltered first sample of said broad band light beam; measuring thefiltered second sample of said broad band light beam; comparing theoutputs of said first sample and said second sample; producing an errorindication in response thereto; moving the central frequency of thebroad band light beam towards the desired centre frequency means as aresult of the error indication.

The invention is further explained, by way of example, by the followingdescription, taken in conjunction with the appended drawings, in which:

FIG. 1 is a schematic diagram of an apparatus, constructed according toa first embodiment of the present invention employing a broad band lightsource.

FIG. 2 is a graph illustrating the band occupied by the broad band lightbeam and the distribution of the narrow band filters around the centraldesired frequency.

FIG. 3 is a block diagram showing a first method of employing thedetected output of the two narrow band filters of FIGS. 1 and 2 togenerate an error signal and a first way of altering the centralfrequency of the broad band light source.

FIG. 4 is a block diagram showing a second method of employing thedetected output of the two narrow band filters of FIGS. 1 and 2 togenerate an error signal and a second way of altering the centralfrequency of the broad band light source.

FIG. 5 is a block diagram again showing the second method of employingthe detected output of the two narrow band filters of FIGS. 1 and 2 togenerate an error signal and a third way of altering the centralfrequency of the broad band light source.

FIG. 6 is a schematic diagram of an apparatus, constructed according toa second embodiment of the present invention employing a pulses broadband light source.

FIG. 7 is a graph showing exemplary reflected light pulses within theapparatus of FIG. 6.

And

FIG. 8 is a block diagram of the apparatus used in the embodiment ofFIG. 6 to generate an error signal from the single photo detector andfurther illustrates one method of controlling the centre frequency ofthe pulsed broad band light source.

Attention is first drawn to FIG. 1, showing a graph of the spectrum of abroad band light source 10 in its ideal desired central position,together with the disposition of the first 28 and second 30 spectralnarrow band filters thereabout.

The first 28 and second 30 spectral narrow band filters are spaced atroughly equal distances below and above the desired frequency 36 of theoutput 38 of the light source 10. The equidistancing does not have to beso, and the present invention can be worked with the desired frequency36 anywhere between the frequencies of the two spectral narrow bandfilters 32 38. In this example of the invention, the first 28 and second30 narrow band spectral filters are positioned approximately at the 3 dbdown points 40 of the output 38 of the light source 10. The first 28 andsecond 30 narrow band spectral filters can be at other distances fromthe desired central frequency 36 and need not necessarily be positionedat or near the 3 db down points 40.

Attention is next drawn to FIG. 2, showing a first way in which theoutputs of a first spectral photodetector 32 and a second spectralphotodetector 34 can be used to control the centre frequency of thelight source 10. The first spectral photodetector 32 is coupled to afirst photodetector amplifier 42 and thence to one side of a summingdevice or junction 44. The output of the second spectral photo detector34 is coupled to a second photodetector amplifier 46 and thence to theother (opposite sign) side of the summing device or junction 44. Thesumming device or junction 44 makes the difference (amplified) betweenthe outputs of the first 32 and second 34 spectral photodetectors andprovides that output as input to a difference amplifier 48 which drives,in this example, a current source 50 for the light source 10. Thefeedback loop is closed by light from the light source 10 being coupledback into the first 32 and second 34 spectral photodetectors. Should thecentral frequency of the light source 10 drift, the difference amplifier48 experiences an error voltage from the summing device or junction 44which drives the amount of current provided by the current source 50 ina direction which brings the central frequency of the light source 10back towards the desired frequency 36. The feedback loop may be providedwith a compensating filter to ensure its stability, as is well-known.

Attention is next drawn to FIG. 3, showing a second way in which theoutputs of the first 32 and second 34 spectral photodetectors can beemployed to correct the frequency of the light source 10. Like numbersdenote like items. The output of the first photodetector amplifier 42 iscoupled as the analogue input to a first analogue to digital converter52. The output of the second photodetector amplifier 46 is coupled asthe analogue input to a second analogue to digital converter 54. Thedigital outputs of the first analogue to digital converter 52 and thesecond analogue to digital converter 54 are coupled as inputs to amicroprocessor 56 which divides the measured size of the output of thefirst spectral narrow band filter 28 by the size of the measured outputof the second spectral narrow band filter 30. The result of thatdivision is provided as digital input to a digital to analogue converter60, whose analogue output is provided to the difference amplifier 48which, in this example, controls a voltage source 62 which controls thevoltage fed to the light source 10. Whenever the light source 10 driftsaway from the desired central frequency 36, the microprocessor 56 notesa change in the ratio between the amplitudes of the outputs of the first42 and second 46 photo detector amplifiers and generates an error outputwhich drives the difference amplifier 48 to deliver a voltage whichpulls the central frequency of the light source 10 back towards thedesired central frequency. The function can equally be implemented insoftware using analog to digital and digital to analog converters and amicroprocessor, as previously indicated.

The microprocessor 56 might equally work in a step mode or in any otherway in which a servo control may be implemented.

Attention is drawn to FIG. 4, which shows another way in which the lightsource 10 can be controlled to stay close to the desired centralfrequency 36. Everything is otherwise as shown in FIG. 3, with theexception that the difference amplifier 48 drives a heating element 64within an enclosure 66 which contains the light source 10. If theenclosure is too hot, the difference amplifier 48 delivers less energyto the heating element 64 and the enclosure 66 cools down to bring thefrequency of the light source 10 back towards the desired frequency 36.On the other hand, if the temperature is too low, the differenceamplifier 48 provides more energy to the heating element 64 to warm upthe enclosure 66 to bring the frequency of the light source 10 backtowards the desired frequency 36. The heating element 64 may also bereplaced with a bidirectional heat pump such as a thermoelectric device.

Furthermore, a second similar feedback loop can be used to measure thesum of the two photodiodes and adjust the current to keep the power ofthe light source 10 constant.

These are just two examples of the way in which the frequencycontrolling servo mechanism may be implemented and three examples of howthe output frequency of the light source 10 can be controlled. In fact,all that is necessary is that some style of servo mechanism is present,is stable, and can control the frequency of the light source 10. Thepresent invention envisages that there is the possibility of controllingthe frequency of the light source 10 by other means such as magnetic,mechanical adjustment, and so on. Moreover, a single analog-to-digitalconverter, designed to measure directly the ratio between a first signalinput and a second reference input voltage or current may be used inplace of the two converters.

Attention is drawn to FIG. 5, showing a first apparatus which can beconstructed according to the present invention. Like numbers mean likeobjects. For this embodiment, at least two narrowband filters are used,each spaced at opposite sides of the central frequency.

The light source 10 is modulated by a pulse generator 68 to producepulses of light which have a very short duration and a repetition periodwhich may be longer than the time taken for the pulse of light to traveldown a fibre optic cable 14 to a sensor 16 and back to the top of thefibre optic cable 14. First and second isolators 70, 72 are provided toensure unidirectionality of travel of light, and a polarisationscrambler 74 is provided in the line from the light source 10 to ensurethat there is no bias in the polarisation of the light in the system.

A very broad band mirror 76 receives part of the light pulse generatedby the light source 10 and reflects it back through a first coupler 12to a second coupler 26. The second coupler 26 couples some of the lightto a first port 78 which comprises a plurality of narrow band reflectors80A, 80B and 80C. Each of the narrow band reflectors reflect a portionof the light that falls upon it, and allows a further portion to passthrough. The narrow band reflectors 80A, 80B, 80C are thus onlyreflective in the part of the spectrum where they are intended to beactive, and are substantially transparent otherwise. Such narrow bandreflectors may be chosen to consist of fibre Bragg gratings, whichfeature low insertion loss, can easily be designed with well designedspectral characteristics, may be tuned to their central frequency, andare easily available commercially. A delay line 82 may precede eachnarrow band filter 80A, 80B, 80C. A second port 84 to the second coupler26 contains a broad band reflector 86.

As the source 10 emits a light pulse, the first coupler 12 couples aportion of the light pulse into the optic fibre line 14 and anotherportion into the very broad band reflector 76. The first coupler 12 thencouples the reflection from the very broad band reflector 76 into thesecond coupler 26 which couples a portion of the energy into the chainof narrow band reflectors 80A, 80B, 80C and into a second port 84 to bereflected by broad band reflector 86.

Attention is also drawn to FIG. 6, which will be of assistance with thefollowing explanation. FIG. 7 is a graph of pulses as experienced by thehigh speed photodetector 88.

A first group of four pulses 90 92 94 96 arise from reflection by thevery broad band reflector 76 of the light pulse.

A first pulse 90, being the first to arrive at the single photodetector, is the result of the reflection from the broad band reflector86 back through the second coupler 26. A short time later, as a resultof a double traverse through the first delay line 82A, a second pulse 92appears. This is the result of the emitted pulse from the broad bandlight source now bouncing off the narrow band reflector 80A back to thesingle photo detector. A short time after that, after a dual traversethrough the first delay line 82A and the second delay line 82B, a thirdpulse 94 appears, which is due to reflection from the second spectralnarrow band filter or reflector 80B. Likewise, a short time afterwards,a fourth pulse 96 appears which is the result of reflection from thethird spectral narrow band filter or reflector 80C. A further set ofpulses appears at a later time, separated by the round trip time downand up the fibre optic cable or line 14. The time scale in the centre ofthe graph which is FIG. 7 separating the first set of pulses 90 92 94 96from the following set 91 93 can be in the order of fractional seconds,whereas individual pulses endure for microseconds or less. This must betaken into account when looking at FIG. 7. The second set of pulses areused for measurements of the pressure in the sensor 16 by finding theratio of the amplitudes of the broad band pulse 91 from the transduceror sensor 16 with the amplitude of the narrow band filtered pulse 93from the transducer or sensor 16.

Attention is drawn to FIG. 7, showing how the output of the high speedsingle photodetector 88 is provided as the analogue input to a highspeed analogue to digital converter 98, which provides its digitaloutput to a microprocessor 100. The microprocessor 100 separates out theinstant pulses 90, 92, 94, 96 from one another by noting the greateramplitude of the first pulse 90, or by noting the time of the pulse orpulses, and counting the order of arrival or noting the time and/orsequence of the subsequent pulses 92 94 96 to establish which pulse iswhich. The microprocessor 100 measures the amplitude of each individualpulse 90 92 94 96 that it receives, and averages the amplitudes of thepulses from two of the spectral narrow band filters over many samples.In one embodiment, the pulses used are those reflected from filters 80Aand 80C.

The microprocessor 100, provides the averaged measurements as a steadydifference signal or ratio, as earlier described, on the digital inputto an error digital to analogue converter 102 whose analogue outputdrives a difference amplifier 48 which can drive the voltage or currentdelivered by the pulse generator 68 driving the light source 10.Equally, the difference amplifier can drive the temperature of anenclosure as earlier indicated (alternatively including a secondfeedback to stabilize power, as also mentioned).

Finally, attention is drawn to FIG. 8, showing another apparatus,constructed according to the present invention, employing a broad bandlight source which is substantially temporally continuous.

A broad band light source 10 provides temporally continuous broad bandlight to a first coupler 12 which couples broad band light to an opticfibre 14, which can be very many kilometres long and can descend into ahostile environment such as an oil, gas or other hydrocarbon well. Atthe far end of the optic fibre 14 is an interferometric sensor 16 whichis designed to measure temperature, flow rate, chemical property,pressure or strain. The sensor 16 can be similar to that used in U.S.Pat. Nos. 6,069,686 and 5,963,321.

The broad band light is returned back along the optic fibre 14 to thefirst coupler 12 which couples it into a beam splitter 18. The beamsplitter 18 splits the broad band return beam from the sensor 16, andfeeds it to measuring equipment 19, in the form of one or more sub beams20. Each sub-beams 20 can pass through one or more filters 24 and thenceon to a respective photo detectors, 22. The exact contents of themeasuring equipment 19 is not of particular concern to the presentinvention, being one of many ways in which light, from the sensor 16,can be analysed to obtain the value of a parameter being measured by thesensor 16.

The apparatus comprises a second coupler 26 which receives a portion ofthe light from the broad band light source 10 via the first coupler 12.The second coupler 26 divides the light between a first spectral narrowband filter 28 and a second spectral narrow band filter 30. The lightwhich traverses the first spectral narrow band filter 28 strikes a firstspectral photodiode 32 and the light which traverses the second spectralnarrow band filter 30 is measured using a second spectral photodiode 34.The output of the photodiodes 32 and 34 can then be used to stabilizethe output of the light source 10.

As discussed earlier, although FIG. 6 shows three narrowband reflectors,the invention only requires that two narrowband reflectors be used, oneon each side of the central frequency.

It is understood that for the embodiments of FIGS. 1 and 6, thetechnique, including at least two narrowband reflectors, may be usedwith any sensor and acquisition system, provided that the narrowbandreflectors receive the light emitted by the light source.

1. An apparatus for processing signals from a remote optic sensor, saidapparatus including means for supplying a broad band light beam for usein interrogating the optic sensor, said apparatus comprising: a firstnarrow band filter and a second narrow band filter, said first andsecond narrow band filters having respective centre frequencies spacedabout the desired centre frequency of the broad band light source;coupling means for coupling a first sample of said broad band light beamthrough said first narrow band filter and a second sample of said broadband light beam through said second narrow band filter; first means tomeasure the filtered first sample of said broad band light beam; secondmeans to measure the filtered second sample of said broad band lightbeam; comparison means to compare the outputs of said first means andsaid second means and to produce an error indication in responsethereto; and means to employ said error indication to apply a correctionto said means to produce said broad band light beam to move the centralfrequency of said broad band light beam towards said desired centrefrequency.
 2. An apparatus, according to claim 1, wherein said first andsecond means to measure comprise a single measurement path and meanstemporally to separate measurement activity for said first filteredsample and for said second filtered sample.
 3. An apparatus, accordingto claim 2, wherein said means to supply a broad band light beam istemporally discontinuous, wherein said first means to measure isoperative to measure only when a first sample of the broad band lightbeam is present for measurement, and wherein said second means tomeasure is operative to measure only when a second sample of the broadband light beam is present for measurement.
 4. An apparatus, accordingto claim 2 or claim 3, wherein said first narrow band filter comprises afirst narrow band reflector, wherein said second narrow band filtercomprises a second narrow band reflector, and wherein said first andsecond narrow band reflectors are separated by a delay line, operativeto cause said first filtered sample and said second filtered sampletemporally to be separated.
 5. An apparatus according to any one ofclaims 2, 3 or 4, wherein said means for coupling said first and saidsecond samples of said broad band light beam comprises a broadbandreflector, said broadband reflector being operative to reflect a mainsample of said broad band light beam to said first narrowband filter andto said second narrow band filter, said first narrow band filter beingoperative to reflect said main sample as said first sample of said broadband light beam, and said second narrow band filter being operative toreflect said main sample as said second sample of said broad band lightbeam.
 6. An apparatus, according to claim 1, wherein said means tosupply a broad band light beam is substantially temporally continuous,and wherein said first and second means to measure are alsosubstantially temporally continuous.
 7. An apparatus, according to anyone of the preceding claims, wherein said cental frequency of the broadband light beam is moveable to coincide with said desired centralfrequency.
 8. An apparatus, according to any one of the precedingclaims, wherein said error signal is a difference signal.
 9. Anapparatus, according to any one of claims 1 to 7, wherein saidcomparison means is operative to take the ratio between the output ofthe first means to measure and the output of the second means tomeasure, and to generate a corrective output error indication if theratio changes.
 10. An apparatus, according to any one of the precedingclaims, wherein said means to employ said error indication to apply acorrection to said means to produce said broad band light beam to movethe central frequency of said broad band light beam is operative tocontrol the current through the means for supplying a broad band lightbeam.
 11. An apparatus, according to any one of claims 1 to 9, whereinsaid means to employ said error indication to apply a correction to saidmeans to produce said broad band light beam to move the centralfrequency of said broad band light beam is operative to control voltageapplied to the means for supplying a broad band light beam.
 12. Anapparatus, according to any one of claims 1 to 9, wherein said means toemploy said error indication to apply a correction to said means toproduce said broad band light beam to move the central frequency of saidbroad band light is operative to control the temperature of the meansfor supplying a broad band light beam.
 13. An apparatus, according toany one of the preceding claims, wherein said broad band light beamprovides illumination for an optical sensor on the distal end of a fibreoptic line in a hydrocarbon well.
 14. An apparatus, according to claim13, wherein said optical sensor is one of, a pressure sensor, a flowrate sensor, a temperature sensor, a chemical property sensor, and astrain sensor.
 15. A method for stabilizing a broad band light source ina system used for processing signals from a remote optic sensor, saidmethod including the steps of: supplying a broad band light beam for usein interrogating the optic sensor; providing a first narrow band filterand a second narrow band filter, said first and second filters havingrespective centre frequencies spaced about the desired centre frequencyof the broad band light source; coupling a first sample of said broadband light beam via said first narrow band filter and a second sample ofsaid broad band light beam via said second narrow band filter; measuringthe filtered first sample of said broad band light beam; measuring thefiltered second sample of said broad band light beam; comparing theoutputs of said first sample and said second sample; producing an errorindication in response thereto; and moving the central frequency of thebroad band light beam towards the desired centre frequency as a resultof the error indication.
 16. A method, according to claim 15, includingthe steps of providing a single measurement path for said first andsecond means to measure, and temporally separating measurement activityfor said first filtered sample and for said second filtered sample. 17.A method, according to claim 16, including the steps of: suppling saidbroad band light beam in a temporally discontinuous manner; operatingsaid first means to measure only when a first sample of the broad bandlight beam is present for measurement; and operating said second meansto measure only when a second sample of the broad band light beam ispresent for measurement.
 18. A method, according to claim 15 or claim 16for use where said first narrow band filter comprises a first narrowband reflector, where said second narrow band filter comprises a secondnarrow band reflector, and where said first and second narrow bandreflectors are separated by a delay line, operative to cause said firstfiltered sample and said second filtered sample temporally to beseparated.
 19. A method, according to any one of claims 16, 17 or 18,including the steps of: employing a broadband reflector to reflect amain sample of said broad band light beam to said first narrowbandfilter and to said second narrow band filter; reflecting said mainsample from said first narrow band filter as said first sample of saidbroad band light beam; and reflecting said main sample from said secondnarrow band filter as said second sample of said broad band light beam20. A method, according to claim 15, including the steps of: supplingsaid broad band light beam in a substantially temporally continuousmanner; and operating said first and second means to measure in asubstantially temporally continuous manner.
 21. A method, according toany one of claims 15 to 20, including the step of moving said centalfrequency of the broad band light beam to coincide with said desiredcentral frequency.
 22. A method, according to any one of claims 15 to21, wherein said error signal is a difference signal.
 23. A method,according to any one of claims 15 to 21, wherein said step of producingan error indication includes the steps of: taking the ratio between theoutput of the first means to measure and the output of the second meansto measure; and generating a corrective output error indication if theratio changes.
 24. A method, according to any one of claims 15 to 23,wherein said step of moving the central frequency of the broad bandlight beam towards the desired centre frequency as a result of the errorindication includes the step of controlling the current through themeans for supplying a broad band light beam.
 25. A method, according toany one of claims 15 to 24, wherein said step of moving the centralfrequency of the broad band light beam towards the desired centrefrequency as a result of the error indication includes the step ofcontrolling voltage applied to the means for supplying a broad bandlight beam.
 26. A method, according to any one of claims 15 to 24,wherein said step of moving the central frequency of the broad bandlight beam towards the desired centre frequency as a result of the errorindication includes the step of controlling the temperature of the meansfor supplying a broad band light beam.
 27. A method, according to anyone of claims 15 to 26, including the step of employing said broad bandlight beam to provide illumination for an optical sensor on the distalend of a fibre optic line in a hydrocarbon well.
 28. A method, accordingto claim 27, wherein said optical sensor is one of, a pressure sensor, aflow rate sensor, a temperature sensor, a chemical property sensor, anda strain sensor.